Rework boxes.  All Items now have a box, (a strategy for accessing
contents), as well as a parent Item, there's a conventional {root}
Item with box=DirComparator and comparators are now pure strategies -
that is, no properties, no instances, using the class directly.

The purpose here was to prepare the way for leaner memory and file
descriptor requirements.  Note that at this point Items grows without
bounds.